In a tire running test apparatus disclosed in PTL 1, for example, a belt wound around a pair of drums moves side to side during the tire test or deviates from the appropriate position in some cases. Thus, the tire running test apparatus controls the belt position while the angle at which the drums rock is made variable after the belt position has been measured.
In a control system such as this tire running test apparatus, the belt, which is a control target, may move in the direction of a command value after firstly moving in the inverse direction at the initial timing of the control due to effects such as the effect of the elasticity of the belt (portion indicated by A in FIG. 1). Such a system that responds so that an output is firstly deflected in an inverse direction at the time of control is called an inverse response system. If the inverse response system has linear dynamic characteristics, the inverse response system is a non-minimum phase system that has a transfer function including unstable zeros.
As illustrated in FIG. 2, when illustrated in a block diagram, the inverse response system can be separated into a minimum phase system P(s) and unstable zeros Z(s).
Here, a polynomial is referred to as a minimum phase polynomial when the real parts of all the roots that satisfy the polynomial=0 are negative. A system that is a linear system and in which both the denominator polynomial and the numerator polynomial of a transfer function are minimum phase polynomials is referred to as a minimum phase system. In other words, a minimum phase system is a stable system expressed by a transfer function in which all the zeros (roots of numerator polynomial=0) and the poles (roots of denominator polynomial=0) are stable (real parts are negative). Meanwhile, unstable zeros are zeros that are unstable and include a positive real part and a system expressed by a transfer function including unstable zeros is referred to as a non-minimum phase system. When unstable zeros, which are slower than stable poles, are included, the non-minimum phase system exhibits inverse response characteristics. For separation of the inverse response system into a minimum phase system P(s) and unstable zeros Z(s), when not all the unstable zeros are integrated into Z(s) but at least one of the unstable zeros is integrated into Z(s) and the remaining unstable zeros are left in P(s), P(s) does not become a minimum phase system but becomes a non-minimum phase system.
Examples of a technology for controlling such an inverse response system include a device disclosed in PTL 2. A device for suppressing pulsation of an electric motor disclosed in PTL 2 performs Fourier-transform on pulsation detection values of an electric motor driven by an inverter, detects pulsation components of an appropriate frequency in the form of two Fourier coefficients, and performs learning control using a learning controller so that the pulsation is suppressed. Thus, the device for suppressing pulsation suppresses pulsation by superimposing the learned pulsation compensation current on a d-axis current command value, a q-axis current command value, or both d-axis and q-axis current command values of a rotating system of coordinates in a vector control. In the device for suppressing pulsation, the learning control system for pulsation suppression with detection of the two Fourier coefficients is formed in a complex vector plane and the device includes means for calculating the pulsation compensation current.